Left ventricular function was studied in patients with mitral valve prolapse using Tc99m gated equilibrium radionuclide cineangiography. Although ejection fraction at rest was normal for those subgroups of mitral valve prolapse patients with and without mitral regurgitation. The average ejection fraction with exercise for both groups was below normal. In patients with mitral prolapse, and no mitral regurgitation, there is a subgroup with diminished left ventricular functional reserve. In those patients with mitral regurgitation, decrease in left ventricular function may represent, in part, the effects of chronic volume overload.